August 5, 2014

Juvenile Whale Sharks Tracked In The Red Sea

The largest fish in the world, at more than 30 feet long, are the whale sharks. Researchers have gained a rare look into the world of these behemoths with a newly-discovered aggregation of juvenile whale sharks off the coast of Saudi Arabia.

An international collaboration of scientists from the Woods Hole Oceanographic Institution (WHOI), the King Abdullah University of Science and Technology (KAUST) and the Massachusetts Division of Marine Fisheries tagged whale sharks at the site to track their movements.

The team performed research work in 2009, finding hundreds of juvenile whale sharks gathering on coral reefs off the central coast of the Saudi Arabian Red Sea near Al-Lith.

"The fact that there were so many whale sharks in such a small area gave us an opportunity to begin an unprecedented study to answer some of the basic questions," said Simon Thorrold, a biologist at WHOI. "The discovery of the site provides a window into the movements and ecology of the species in a region that we were not necessarily expecting to see them in such high numbers."

From 2009 to 2011, three types of satellite transmitting tags were used to track the movements of 47 whale sharks. Placed just below the dorsal fins, the tags measured temperature, depth, and light levels of the waters the fish swim in for months before popping off. The tags then float to the surface and beam the data back to computers on the shore via ARGO satellites.

The data revealed that, as a whole, the whale sharks frequently made deep dives of at least 1,640 feet. Three particular individuals, however, made dives below 3,281 feet. The maximum recorded dive during the study period reached a depth of 4,462 feet.

While the majority of the tagged sharks remained in the Red Sea during the study period, some did leave for the Indian Ocean. The researchers suggest this could be to follow an abundant food supply related to seasonal upwelling. Scientists believe the main diet of whale sharks to be zooplankton, along with algae, small fishes, fish eggs and cephalopods. The sharks' diet is known to vary both seasonally and geographically.

"Interestingly, while some individuals that we tagged left the Red Sea and headed into the Indian Ocean, most remained relatively close to where they were tagged, suggesting that the area represents a critical juvenile habitat for this population," Thorrold added.

The research team did not observe any adult whale sharks at the Al-Lith site, which they suggest might be a "staging ground" for juveniles before they move on to regional aggregations of larger sharks. Scientists have identified 12 such aggregation sites globally, thus far.

"While all other juvenile whale shark aggregations are dominated by males, we found a sex ratio of 1:1 at the site in the Red Sea. The presence of so many female juvenile sharks may be of considerable significance to the global whale shark populations," Thorrold said.

In 2000, the International Union for Conservation of Nature (IUCN) listed the whale shark as "vulnerable." Their populations do not bounce back quickly because, like many other sharks and rays, the whale shark is slow to sexually mature and typically has small litters of pups.

The team hopes their findings will show the need for further research on whale sharks. They would also like to see a multinational, cooperative effort to conserve populations in both the Red Sea and the Indian Ocean.

"Our research on whale sharks in the Red Sea clearly shows that much remains to be learned about the species," said Greg Skomal, a Massachusetts Division of Marine Fisheries biologist and WHOI adjunct scientist. "This newly discovered aggregation off Saudi Arabia likely plays an integral role in the natural history of this species in this region and, perhaps, well beyond."

"Eventually this work will tell us about where whales sharks are spending their time throughout their lives, where they're feeding, where they're breeding and where they're giving birth," added Thorrold. "Knowing where they go at different times of the year is critical to designing effective conservation strategies for the species."